There are a lot of moving parts here, involving Syria, Russia, USA, Iran, Palestine, Hezbollah, Israel, Saudi Arabia, UK, France, etc. ...
Keeping it simple, if we say there are two sides, "our side" and "the other side", then from our side's POV the other side has used chemical weapons against its own people and that needs to be punished/prevented from happening again; from the other side's POV we are looking for excuses to bomb them and weaken the axis of power that (among other things) makes Israel feel threatened. So Corbyn, in getting involved, is drawn back towards the Jewish debate, and what side does he find himself on again?